I don't think it proves it though. The code that awards the achievement and the achievement progress tracker code may be different.

Well yeah, there's only one way to definitively prove something in game, which is by doing it.

However, If I were exalted with all the goblins on my main, and my alt wasn't, if the achievement progress on my alt showed that I already reached the exalted requirement with the goblins...I would feel pretty sure about continuing progress on an alt.